Anatomy of a class: instance variables, constructors and encapsulation

Learning objectives

Write a public class with private instance variables, and explain how encapsulation, with private data reached only through public methods, protects the state of every object

Explain that the state of an object is the current values of its instance variables, which gives a has-a relationship, and that each object keeps its own copy of those variables

Write constructors, including overloaded ones, that give every instance variable an initial value, taken from the parameters or from sensible defaults

State the default values Java gives instance variables when a class has no constructor written (0, 0.0, false and null), and explain what becomes of that default constructor once any constructor is written

Write accessor methods that return the value of an instance variable and mutator methods that change it, applying any validation the specification requires

Explain why a constructor that receives a mutable object as a parameter should store a copy of it rather than the reference it was given